Depends entirely on the girls. The pinnacle is obviously a lesbian double where the girls are kissing and sucking each others titties and eating each other out. Nothing beats this.
If you can't get a lesbian double then you at least want one girl to be kissing you or letting you suck her titties or touching the other girl in some way while you're fucking the other one.
The worst, and I've had a couple of these is where one girl just sits down the end of the bed while you fuck the other. Then they switch but one always remains out of reach and away from the action. Complete waste of a double if this happens.